COLOUR VISION
colour blindness
See
colour vision deficiency.
colour vision
The ability to see and distinguish the
different parts of the colour spectrum,
which consists of electromagnetic radi-
ation (energy waves) with a range of
wavelengths between about 400 and
700 nanometres (millionths of a milli-
metre). Different wavelengths trigger
nerve signals in the
retina
(the light-
sensitive layer of cells at the back of the
eye); these signals pass to the brain and
are interpreted as violet, indigo, blue,
green, yellow, orange, and red.
RETINAL FUNCTION
As light falls on the retina, it strikes
light-sensitive
cells
called
rods
and
cones. The rods can detect all visible
light, but only the cones can distin-
guish colour. There are three types of
cone: red-sensitive, blue-sensitive, and
green-sensitive. Each of these types of
cone responds more strongly to a par-
ticular part of the light spectrum. The
cones are most concentrated in a cen-
tral area of the retina called the
fovea;
for this reason, colour vision is most
accurate for objects that are viewed
C
COLOUR VISION
Light, consisting ofradiation ofvarious wavelengths, is
focused on the retina. At the back ofthe retina, light-sensitive
rod and cone cells are stimulated to emit electrical impulses.
The impulses then travel through cells called bipolar cells to a
layer called the ganglion cells, where some initial processing
occurs, before passing to the brain via the optic nerve.
Location ofcolour-sensitive cells
The rods and cones are located at the back of
the retina. Behind them is a darkly pigmented
layer of cells, which reduces light scattering.
Colour vision depends on the cones, which
are concentrated in an area called the fovea.
Key
— — — — —
Blue-sensitive cones
Green-sensitive cones
...................
Red-sensitive cones
Colour response of cones (right)
There are three classes of cone. One ofthese
(red-sensitive) responds best to light of long
wavelengths; another (blue-sensitive) to light
of short wavelengths; and the third (green-
sensitive) to intermediate wavelengths.
i
Light
400
450
500
550
600
650
700
750
Wavelength (nanometres)
Response to white light
White light consists of a mixture
of all wavelengths (colours),
and ittherefore stimulates all
three classes of cone to signal
equally. This pattern of
response is interpreted as
whiteness bythe brain.
Key
G
Green-sensitive
R
Red-sensitive
B
Blue-sensitive
Response to red light
Light with a long wavelength (red
light) produces a strong response
from red-sensitive cones, a weak
response from
blue-sensitive
cones, and an intermediate
response from green-sensitive
cones. This pattern ofsignalling
is interpreted as the colour red by
the brain.
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